This invention relates to the anchoring of rock bolts and cables and, more particularly, relates to a method and apparatus for permanently anchoring ground anchor cables and rock bolts in drill holes in rock.
Anchor cables, tie rods and rock bolts have long been used as anchoring elements to strengthen or stabilize mine roofs. Such anchoring elements may be a length of wire or cable, reinforcing bar, or may be rock bolts of various shapes, configurations and sizes.
In order to improve the contact between the anchoring elements and the mine roof, the elements are often fixed or anchored in the drill holes at their inner ends or over substantially their entire lengths. In one method of fixing bolts and cables, the anchoring is achieved by means of a reactive composition which hardens around the anchoring element. For mine roof supports, compositions are needed which harden and rapidly attain maximum strength. Compositions which have been used in the past include inorganic cement compositions, grouts and synthetic resins which have been introduced into drill holes through a feed pipe, or in cartridged form.
In the case of inorganic cement compositions, a prepared hydraulic cement mortar is pumped as a slurry from a container into a drill hole after the anchoring element has been placed in the hole. Alternatively, the element can be driven into the hole filled with mortar which has been pumped in or has been placed in the hole in cartridges.
Various methods and apparatus for anchoring rock bolts and cables in drill holes in mine roofs using inorganic cement compositions are described in U.S. Pat. Nos. 2,233,872, 2,313,310, 2,667,037, 2,930,199, 3,108,422, 3,227,426, 3,326,004, 3,363,422, 3,371,494, 3,436,923, 3,494,134, 3,572,956, 3,604,213, 3,735,541, 3,986,536, 4,126,009, 4,179,861, 4,229,122, 4,252,474, and 4,289,427.
The prior art methods and apparatus have a number of disadvantages. The inorganic cement composition or grout is usually prepared as a slurry which is kept in a container prior to being pumped into the drill hole. The prepared mixture must be agitated and must be pumpable, and thus contains a relatively large quantity of water. This large amount of water requires that the injected material, once injected, must be prevented from escaping from the drill hole by means of a plug, cover or collar plate. The prepared mixture must not be allowed to set in the container, thereby limiting the amount that can be prepared to that which can be used within a short period of time. The same limitations apply when a cement slurry and water of hydration are mixed just prior to injection, or are mixed in the drill hole. Cables and bolts which are fixed in drill holes with grout frequently have a bond with the surrounding earth or mine roof which is friable and which can break down with vibrations such as caused by drilling and blasting. In addition to these disadvantages, the equipment for preparing, storing and injecting the cement compositions is relatively elaborate and thus expensive.
In distinction to these "wet methods", U.S. Pat. No. 4,498,817 discloses the use of a grit moistened with water.
This method has the disadvantage of providing only a mechanical bond, of requiring the installation of a plate to prevent particulate material from falling from the drill hole, and of settling of the material with time and vibration requiring regular tightening of the roof bolts.